1. Field of the Invention
This invention relates to a voltage generator, and particularly to a voltage generator for generating a predetermined voltage through the use of electromagnetic induction, based on a received signal.
2. Description of the Related Art
A voltage generator has been applied to various devices. Of cards (hereinafter called "IC cards") each having a semiconductor integrated circuit incorporated therein, which have been widely used as portable storage media in particular, it is known that the voltage generator is used in a card (hereinafter called "non-contact IC card") capable of performing the transfer of data without having to use a contact terminal for an external signal.
The voltage generator built and used in the non-contact IC card generates a voltage through the use of electromagnetic induction, based on a signal transmitted from the outside of the non-contact IC card, for example. The voltage generated by the voltage generator is used as an operating voltage for other circuits (such as a semiconductor integrated circuit) incorporated into the non-contact IC card. This operating voltage is used as a source voltage or data to be processed by the semiconductor integrated circuit. This type of voltage generator has been disclosed in the following references:
(1) Japanese Patent Application Laid-Open No. Hei 4-23092
(2) Japanese Patent Application Laid-Open No. Hei 10-97601
As is understood from each disclosure referred to above, the voltage generator using electromagnetic induction is constructed so as to obtain a predetermined voltage, based on alternating current signals produced by a coil corresponding to a receiving circuit. Therefore, there may be a case in which when the distance between a coil of an external device (e.g., card reader/writer) and the coil of the voltage generator increases, a voltage or current based on ac signals produced by the coil of the voltage generator is not achieved to a level enough to produce an operating voltage. There has recently been an increasing demand for a voltage generator for generating a voltage having a sufficient level as an operating voltage, regardless of the distance between the external device and the voltage generator.